: Wild-type p53 is a potent tumor suppressor that is activated by DNA damage and other stresses. There has been considerable interest in restoring wild-type p53 function as a therapeutic strategy. This goal has led to the development of the Nutlin-3 (Nutlin), a small molecule that activates wild-type p53 by blocking its interaction with MDM2, the primary negative regulator of p53 activity in cells. Notably, Nutlin activates p53 through a non-genotoxic mechanism, and thus its use as a therapeutic agent may spare tissues of deleterious side-effects associated with common DNA damaging drugs. Effective use of Nutlin requires that its effects on cells be fully understood. We have examined the response of various p53 wild-type cell lines to transient Nutlin treatment. We find that p53 activation by Nutlin can promote growth arrest or apoptosis in cells dependent on activation of survival pathways, and we have identified a candidate survival factor that protects cells from Nutlin-induced apoptosis. We also find that Nutlin has surprising effects on cytoskeletal organization and control of DNA endoreduplication. The purpose of this grant is to determine the effects of Nutlin-mediated p53 activation on these various cellular responses.